Embodiments of the present invention relate generally to packet-based wireless networks. More specifically, embodiments of the invention related to systems and methods for transmitting packets in a packet-based wireless network by concatenating a plurality of data packets into a single packet transmission.
As wireless networks become increasingly popular, there is an ever greater need to provide higher data throughput from existing bandwidth allocations. Typically, a wireless network must operate within an assigned band of frequencies. This is usually the case regardless of the type of physical transmission technique utilized. For example, frequency hopping systems usually must only hop to frequencies contained within a fixed range of frequencies. Similarly, spread spectrum systems must remain within the bounds of a well-defined frequency band. Of course, frequency division multiplexing systems also are confined to fixed bandwidths. Thus, a fundamental performance goal in the implementation of wireless networks is to provide as much data throughput as possible given a particular bandwidth allocation.
Packet-based wireless systems typically transmit and receive one packet at a time over the wireless medium. Such transmissions typically require header information to be appended to a packet of data, such as an Ethernet packet, so that medium access control (MAC) and physical layer (PHY) functions can be performed for a wireless transmission. This added header information is overhead added for wireless transmission and reception.
Consider, for example, the illustrative wireless network 100 of FIG. 1. The wireless network 100 includes an access point (AP) 102 and stations (STAs) 104 and 106. Typically, wireless networks may include numerous APs and STAs, but the simplified wireless network 100 is shown in here for illustrative purposes. Each of the three nodes, AP 102, STA 104, and STA 106, is able to both transmit and receive packets over the wireless medium. Different types of communication may be possible. For example, in one arrangement, all communication may be required to go through AP 102. Thus, if STA 104 wishes to transmit a packet to STA 106, the transmission must first be sent to AP 102, then relayed to STA 106. In another arrangement, STA 104 may communicate directly with STA 106, without involving AP 102. Regardless of the type of communication chosen, a fundamental component is a transmission that involves one node (AP or STA) acting as the transmitter and another node (AP or STA) acting as the receiver of the transmission.
In this example, the wireless network 100 will be assumed to operate according to 802.11 protocols. According to such protocols, Ethernet packets are accepted by the MAC-layer processing of an 802.11 transmitting device, AP 102. The MAC layer processing appends MAC control information to the beginning and the end of the data packet and various control fields to the frame body. The entire MAC frame is then sent to the PHY for processing.
There are various PHY packet formats according to 802.11 protocols. An exemplary format is the PHY packet format for 802.11a. Pictured in FIG. 2. The MAC frame is accepted from the MAC-layer and various control and management information is appended to form a Physical Layer Convergence Protocol (PLCP) Data Unit (PPDU) 200. The PPDU 200 includes a PLCP Preamble (PLCP Preamble), a Signal Field (SiF), a Service Field (SrF), a PLCP Service Data Unit (PSDU), six tail bits (Tail), and pad bits (Pad). The PLCP Preamble is used for receiver synchronization (gain, time and frequency). The Signal Field is used by the receiver to determine the data rate and length of the packet being sent. The Service Field is used by the receiver for descrambling operation. The tail and pad bits complete the Forward Error Correction (FEC) decoding and fill out the available Orthogonal Frequency Division Multiplexing (OFDM) symbols.
The control and PHY management control fields added to the PSDU are overhead that reduces the throughput of the packet data contained in the PSDU. The PLCP preamble occupies 16 μs of time. The Signal Field occupies 4 μs of time. Together this 20 μs of time is the primary overhead in the PPDU.
When the PSDU data size is very large, the extra 16 μs of PLCP preamble may not be much overhead. For example, consider the case of a 1500 byte PSDU with 54 Mbps used as the PHY rate. A PLCP preamble of 16 μs is very little overhead (about 6.6%) to the total time it takes to transmit 1500 bytes. However, a 64 byte PSDU occupies very little time on the air. Hence, the PLCP header adds 50% overhead. The below table summaries the effect of the 16 μs of PLCP preamble overhead on the effective data rate. The effective data rate is computed by dividing the total number of PSDU bits transmitted by the amount of time it takes to transmit.
PSDU Size (bytes)150064PHY Date Rate (Mbps)5454Payload Time (μs)22412Packet Time (μs)24432PLCP Overhead (%)6.650.0Effective Data Rate (Mbps)49.1816.00
Because the efficiency of a transmission in a packet-based wireless network can be increased by decreasing the amount of overhead necessary to transmit a data packet, improved methods and systems are needed that decrease such overhead.